Stiff, 'sun-dried' books - help!

[Siavahda]

I don't know if there's a proper, technical term for this; I've just been calling it 'sun-dried' since I was a kid. But sometimes - I think it's related to heat, although I've never been sure - a book or books will get stiff, so that all the pages bend one way or the other. You can make them bend the other way, and then they stay bent in that direction instead, but they don't lie flat or flip-through normally. This happens equally to softcovers and hardcovers; it's just that with the hardcovers, the covers themselves aren't bending too. Leaving them pressed beneath other, heavier books, or other heavy weights, makes no difference.

 

Does anyone know what this is, and more importantly how to fix it? We recently moved house and I was appalled to discover that, a few weeks in, all my books have become like this. This apartment isn't noticeably warmer than our last, and my bookcase is well away from any windows or radiators in any case. We do think the last apartment was much more humid, but I always thought that humidity was bad for books, not good for them!